Fluorescence is used in medical practice for visualizing certain types of tissue. For this purpose, substances are administered to a patient which selectively accumulate in different types of tissue and provide fluorescent substances in these tissues. When illuminated with suitable light, a fluorescence of the fluorescent substances will be excited, and it is possible to observe the emitted fluorescent light. In an image recorded with the fluorescent light, certain tissues having a higher concentration of the fluorescent substance appear brighter than surrounding tissues containing lower concentrations of the fluorescent substances. For example, when the administered substance concentrates in the a certain tumor tissue, tissue regions affected by the tumor can be visualized and differentiated from the surrounding tissue regions not affected by the tumor.
Treatments to the human body can be performed by using surgical microscopes allowing to perform fluorescence observation. Such surgical microscope typically includes an illumination light filter in a beam path between a light source and an observed object, and an observation light filter in a beam path between the object and an observing eye or a camera detecting images of the object. The illumination light filter allows light having wavelengths suitable for exciting a fluorescence in the object to traverse the filter and to reach the object. The observation light filter allows fluorescent light emanating from the object to traverse the filter and reach the observing eye or camera. The illumination light filter is further configured to block light having wavelengths of the fluorescent light which can traverse the observation light filter, so that substantially only light generated by the fluorescence traverses the observation light filter while illumination light reflected or scattered at the object does not reach the observing eye or camera. The illumination light filter blocks a significant amount of the spectrum of the illumination light so that, if the used light source is a white light source, the illumination light reflected or scattered at the object will not allow to perceive the object with a natural unaltered color impression.
An example of a substance to visualize certain types of tumors is Protoporphyrine IX which is selectively accumulated in tumor tissues by administering 5-Aminoluvelinacid (5-ALA) to the patient.
Protoporphyrine IX is also used for visualizing tumors of the human brain. Herein, it is important for the surgeon to be able to accurately identify the boundary of a tumor in order to remove the tumor without removing surrounding tissues, so that important functions of the brain are preserved.
From U.S. Pat. No. 6,212,425 B1 there is known an optical filter system including an illumination light filter and an observation light filter adapted to allow observation of the fluorescence of Protoporphyrine IX.
When using this light filter system, the healthy tissue is visible in a blue color, tumor tissue is visible in a red color, and boundary regions of the tumor tissue appears in a mixed color, which is described by many people as salmon colored, so that it is relatively easy to identify the boundary of the tumor tissue. However there is a problem, in that details of the healthy tissue, such as blood vessels are not visible since the healthy tissue is only visible as a blue tissue. In order to identify such details of the healthy and/or tumor tissue, it is conventionally necessary for the surgeon to switch from a fluorescence observation mode of the surgical microscope to a normal light observation mode, in which details of the healthy tissue and the tumor tissue are visible while it is not possible to distinguish between the healthy and the tumor tissue. The switching back and forth between the fluorescence observation mode and the normal light observation mode is very demanding for the surgeon.
Thus, it is desirable to be able to visualize the fluorescence of fluorescent regions and to perceive a relatively unaltered natural color impression of the non-fluorescent regions in a same mode of the surgical microscope.